Recently, energy-saving has been actively promoted in terms of environmental measure. For portable electronic equipment using battery, such as a mobile phone, a digital camera, a PDA (portable digital assistant), a notebook computer, a portable multimedia player (for example, MP3 player, optical disc player, etc.), and so on, it is desirable to have a longer battery life. Such portable equipment commonly employs a switching regulator in power system because a compact and high efficiency power system can be obtained using the switching regulator.
A conventional switching regulator generally includes a feedback circuit to stabilize output voltage. In the feedback circuit, an ON/OFF circuit is controlled by changing an input pulse width applied to the ON/OFF circuit. The conventional switching regulator includes an oscillation circuit and a comparator which outputs a control signal to control the oscillation circuit. The comparator compares an output voltage of an error-AMP (amplifier) with a reference voltage.
For example, if the control signal from the comparator outputs is H (high), the oscillation circuit oscillates with a high frequency. If the control signal from the comparator outputs is L (low), the oscillation circuit oscillates with a low frequency which is lower frequency than the high frequency. The operation of the switching regulator will be described more specifically.
FIG. 1 illustrates a conventional switching regulator 100. The switching regulator 100 includes a triangular-pulse generator 10, a first and second reference voltage circuits 11 and 14, bleeder resistances, an error-AMP 12, a comparator 13 and a PWM (pulse width modulation) comparator 15.
The error-AMP 12 inputs an output voltage Vout of the switching regulator 100, which is applied to a load as a load voltage, and a reference voltage Vref1 of the first reference voltage circuit 11. The error-AMP 12 outputs a voltage difference between these two voltages. The comparator 13 inputs the voltage difference and a second reference voltage Vref2 of the second reference voltage circuit 14. The comparator 13 judges whether the difference voltage is higher than the reference voltage Vref2 of the second reference voltage circuit 14.
The PWM comparator 15 outputs a control signal by comparing an output signal of the triangular-pulse generator 10 with the output voltage of the error-AMP 12. The output signal of the triangular-pulse generator 10 is a triangular wave.
FIG. 2 illustrates waveforms showing the operation of the switching regulator 100. The output voltage of the error-AMP 12, ERROR AMP OUTPUT, is being changed by comparing the output voltage of the error-AMP 12 with the triangular wave output from the triangular-pulse generator 10. As a result, the output pulse width of the PWM comparator 15 is controlled. An ON or OFF time of a switching transistor arranged next to the switching regulator 100 is controlled for a corresponding time to the output pulse width of the PWM comparator 15.
In some switching regulators which do not employ a current control mode, a feedback loop from the output of the switching regulator may include a time lag. The feedback speed may not be fast enough to control due to the time lag. The feedback voltage may move to higher or lower voltage than an expected voltage and may not be adjusted to a desired feed back voltage. As a result, the switching regulator may oscillate unintentionally.
Even if a switching regulator employs a current control mode and if a duty cycle of the switching regulator exceeds 50%, a slope compensation circuit may be required. The slop compensation circuit makes a slower rising edge of the output voltage of the switching regulator to avoid a destruction of transistor. However, using the slope compensation circuit, the switching regulator may be larger and complicated.